1. Field of the Invention
The present invention relates to an acceleration sensor for sensing an acceleration based on a variation in capacitance, and to a method of producing same.
2. Description of the Related Art
FIG. 12 is a perspective view of a movable body in a conventional acceleration sensor disclosed in Japanese Patent Application Laid-open No. 8-21852, FIG. 13 is a plan view of the movable body of FIG. 12, and FIG. 14 is a sectional view of an essential portion of the acceleration sensor that incorporates the movable body of FIG. 12.
The acceleration sensor comprises a frame substrate 1, a dielectric movable body 3 having slits 4 therein, connected to the frame substrate 1 by a first beam portion 2a and a second beam portion 2b, a first fixed substrate 5a and a second fixed substrate 5b, both of which are manufactured of an insulating material such as ceramic and between which the frame substrate 1 is interposed, a plurality of first electrodes 6a arranged along the first fixed substrate 5a, and a plurality of second electrodes 6b arranged along the second fixed substrate 5b facing the first electrodes 6a respectively.
A capacitor C is formed by facing first and second electrodes 6a, 6b towards each other, and is electrically connected in series to another capacitor C that is formed by towards each other facing adjacent first and second electrodes 6a, 6b.
When the acceleration sensor thus constructed is under acceleration which acts on the movable body 3 in the direction of the arrow A or B as shown in FIG. 13, the movable body is displaced according to the magnitude of the acceleration in the direction of the arrow A or B. The displacement of the movable body 3 changes the ratio of the dielectric areas of the movable body 3 between the slits 4 to the first electrodes 6a and the second electrodes 6b, thereby changing the capacitance of the capacitor C. The variation in capacitance is output as a voltage that is proportional to the acceleration applied to the movable body 3.
In the acceleration sensor thus constructed, the movable body 3, that is displaced in response to the applied acceleration, has a plurality of slits 4. Therefore, the mechanical strength of the movable body 3 is low and it is likely to be damaged by any impact during manufacturing.
The movable body 3 is displaced in response to the acceleration applied to the movable body 3 against the elasticity of the beams 2a, 2b. Thus in order to displace the movable body 3 a large amount, the movable body 3 needs to have a certain degree of heaviness. The volume of the movable body 3 having the slits 4 is thus increased, thereby making the entire acceleration sensor bulky.
Also, since the fixed substrates 5a, 5b are provided with the electrodes 6a, 6b, the manufacturing process is accordingly complicated.
If, for example, a charge is generated in the second fixed substrate 5b as shown in FIG. 14, for example, a stray capacity C.sub.F is formed between the first electrode 6a and the second fixed substrate 5b. When the capacitance of the stray capacity C.sub.F is substantially large enough to not be negligible in compared to the capacitance of the capacitor C that varies under acceleration, the acceleration sensing accuracy is adversely affected.